Creating a Txt-File and write to it

I want to create a text file then add the text of a TextBox to it. Creating the text file works without any problems with following code:

InitializeComponent();
string path = @"C:\Users\Morris\Desktop\test.txt";
if (!File.Exists(path))
{
    File.Create(path);
}

But I get an error that the file is being used when I try to add the text to the text file. If the file already exist before it run the code I don't get this error and the TextBox.Text is added to the File. I use this code to add the text to the text file:

public void writeTxt()
{
    string path = @"C:\Users\Morris\Desktop\test.txt";
    if (File.Exists(path))
    {
        using (var tw = new StreamWriter(path, true))
        {
            tw.WriteLine(TextBox1.Text);
            tw.Close();
        }
    }
}

Can you help me?